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1

Digitale Medien

The effect of the duration of regrowth on photosynthesis, leaf death and the average rate of growth in a rotationally grazed sward (1988)

PARSONS, A. J. ; PENNING, P. D.

Oxford, UK : Blackwell Publishing Ltd

Grass and forage science 43 (1988), S. 0

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ISSN: 1365-2494

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft

Notizen: A study was made to characterize the effect of the duration of regrowth on the pattern of changes in the major physiological processes involved in the net accumulation of herbage, and so to provide a rational basis for optimizing production under rotational grazing.During regrowth following a severe defoliation, rates of canopy photosynthesis, and so the rate of production of new leaves, increased rapidly but there was a delay before there was a corresponding increase in the rate of leaf death. Although the amount available for harvest (crop live dry weight, W) continued to increase as the duration of regrowth was extended from ‘short’ (12–13 days), through ‘medium’ (19–23 days) to ‘long’ (30-34 days), there was not a continued increase in the average growth rate – the increase in the weight of the crop, (W – Wo), divided by the duration of regrowth, t, In this study, the average growth rate (based on changes in the weight of lamina alone) increased as the duration of regrowth was extended from 12-13 to 19-23 days but changed little as the duration of regrowth was extended from 19-23 days to 30-34 days. In spring and summer, elongating stems increased the average growth rate (of lamina plus stem) up to 30-34 days but the accumulated stem material could not reliably be harvested by sheep.A mathematical model was used to explain why the average growth rate is characteristically insensitive to the duration of regrowth beyond a given minimum duration. For practical purposes, we suggest from the results of this study that regrowths of at least 14 days but less than 28 days will be effective in achieving not only close to the maximum average growth rate of highly digestible material, but also in sustaining a densely tillered, leafy sward which regrows rapidly from severe defoliation and is more reliably harvested (utilized) by sheep.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1365-2494.1988.tb02137.x

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Digitale Medien

Leaf age structure and canopy photosynthesis in rotationally and continuously grazed swards (1988)

PARSONS, A. J. ; JOHNSON, I. R. ; WILLIAMS, J. H. H.

Oxford, UK : Blackwell Publishing Ltd

Grass and forage science 43 (1988), S. 0

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ISSN: 1365-2494

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft

Notizen: The leafage structure of ryegrass canopies and its role in canopy photosynthesis were compared under continuous and rotational grazing by sheep. Under continuous grazing, an increase in the intensity of grazing increased the proportion (by leaf area) of young leaves in the sward. A mechanistic mathematical model was used to demonstrate how this may have arisen, even though it would largely have been the young leaves that were eaten.However, the observations do not confirm the hypothesis that continuously grazed swards have a characteristically greater proportion of young leaves, and so a greater photosynthetic potential, than rotationally grazed ones. The proportion of young leaves increased during regrowth following severe rotational grazing (residual LAI 〈 05) and the photosynthetic potential of the canopy became greater than under continuous grazing.A model of canopy photosynthesis was used to demonstrate that the observed difference in the proportion of young leaves alone was unlikely to account for all the differences in canopy photosynthesis between managements, and further differences in canopy structure were evaluated. Despite the delay in the restoration of leaf area following severe grazing in a rotation, the total photosynthetic uptake of a system involving some 12–13 days regrowth and 3 days grazing exceeded that of a well-utilized continuously grazed sward. Re growths of longer duration led to progressively greater total photosynthetic uptake, though this was not considered synonymous with greater yield.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1365-2494.1988.tb02136.x

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3

Digitale Medien

Use of a model to optimize the interaction between frequency and severity of intermittent defoliation and to provide a fundamental comparison of the continuous and intermittent defoliation of grass (1988)

PARSONS, A. J. ; JOHNSON, I. R. ; HARVEY, A.

Oxford, UK : Blackwell Publishing Ltd

Grass and forage science 43 (1988), S. 0

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ISSN: 1365-2494

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft

Notizen: A previously described model of grass growth was used to analyse the effect of the severity of defoliation and the duration of the subsequent regrowth on the overall balance between photosynthesis, gross tissue production and leaf death and so on the amount that could be harvested/consumed per hectare under intermittent defoliation. Maximum yield per hectare was shown to be achieved whenever the fluctuations in LAI during regrowth and defoliation led lo the same low average LAI that was previously shown to give maximum yield (amount harvested/consumed per hectare) under continuous grazing. Thus, it is suggested that production under both continuous and intermittent defoliation may best be characterized, and diverse managements may be rationalized, on the basis of the average sward state, the average achieved. The analysis leads us to reconsider some widely held concepts of the growth and utilization of grass applied in both agronomic and ecological theory, and the provision of practical guidelines for management.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1365-2494.1988.tb02140.x

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4

Digitale Medien

Seasonal patterns of grass production under cutting or continuous stocking managements (1988)

ORR, R. J. ; PARSONS, A. J. ; TREACHER, T. T. ; [weitere]

Oxford, UK : Blackwell Publishing Ltd

Grass and forage science 43 (1988), S. 0

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ISSN: 1365-2494

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft

Notizen: A comparison was made between the seasonal pattern of net herbage accumulation under cutting and the pattern of intake under continuous variable stocking on a Lolium perenne sward, which was well supplied with nitrogen fertilizer and irrigated to minimize moisture stress.The seasonal pattern of production under monthly cutting showed characteristic asymmetry with a marked peak in production in late May/early June, a rapid decline in June followed by an increase in rate in August and the total amount harvested over the season was 9–28 t organic matter ha−1.By contrast, the seasonal patterns of production on swards grazed by ewes to maintain sward surface heights of 30, 50 and 70 mm all differed fundamentally from that observed under cutting. Production was more evenly distributed throughout the season under these grazing treatments which, it has been proposed, should optimize performance. Total amounts harvested by grazing were 10–84, 10.12 and 8.51 t OM ha−1, respectively. The different patterns of production under cutting and grazing were considered to be associated with differences in the expression of reproductive development in the grass crop and the maintenance of a smaller leaf area per unit ground area under grazing.The results of the study suggest that it is not appropriate to regard the seasonal pattern of net accumulation under a standard cutting regime as representing the potential production of grass under continuous stocking. Moreover, in practice, the more uniform seasonal pattern of production seen in swards maintained on the basis of a controlled sward surface height makes it easier to match the supply of herbage to the appetite of grazing animals as the season progresses.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1365-2494.1988.tb01888.x

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5

Digitale Medien

Use of a model to analyse the effects of continuous grazing managements on seasonal patterns of grass production (1985)

JOHNSON, I. R. ; PARSONS, A. J.

Oxford, UK : Blackwell Publishing Ltd

Grass and forage science 40 (1985), S. 0

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ISSN: 1365-2494

Quelle: Blackwell Publishing Journal Backfiles 1879-2005

Thema: Land- und Forstwirtschaft, Gartenbau, Fischereiwirtschaft, Hauswirtschaft

Notizen: The effects of management on the seasonal patterns of production of grass swards under continuous grazing are examined using a previously described mathematical model. For continuous set-stocking it is observed that, if the leaf area index of the sward at turn-out is too low, there may be a sustained reduction in animal intake throughout the season. Under continuous variable stocking, where the sward is maintained at a constant leaf area index, animal intake is observed to follow broadly a pattern determined by the environmental conditions. It is concluded that grass production, in terms of both gross tissue production and animal intake, is not independent of management. Furthermore, no evidence is found to suggest that the seasonal patterns of production undercutting and grazing are the same.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1111/j.1365-2494.1985.tb01776.x

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6

Digitale Medien

Plight of postdocs (1987)

PARSONS, A.

[s.l.] : Nature Publishing Group

Nature 326 (1987), S. 538-538

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ISSN: 1476-4687

Quelle: Nature Archives 1869 - 2009

Thema: Biologie , Chemie und Pharmazie , Medizin , Allgemeine Naturwissenschaft , Physik

Notizen: [Auszug] SIR-Dennis Murphy in his letter on the plight of British postdocs (Nature 325, 478; 1987) raises some pertinent points. The sentiments he expresses, however, bear the hallmark of one safe in the knowledge that he has security of tenure. The problem of salary grading and structure for research staff ...

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1038/326538b0

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